Can not join two vertices
J only works on faces that already exist, it's basically a shortcut to cut a face between vertices (like the knife tool but just for cutting a face between vertices with a single button press)
You want the F key for this one. It will &quot;fill&quot; an edge between the 2 vertices.
If you want to make a face from 3 or more vertices, select the vertices and hit F to make a face.
F can be used in many different situations, like simply selecting a single edge that has other edges connected to it and boom, instant face. Try it out!